Typically used non-aqueous electrolyte solutions for electrochemical devices such as lithium-ion secondary batteries are produced by dissolving electrolyte salts (e.g. LiPF6, LiBF4) in non-aqueous solvents (e.g. ethylene carbonate, propylene carbonate, dimethyl carbonate).
In the case of using such an electrolyte salt containing fluorine, however, a very small amount of moisture contained in non-aqueous solvents and electrochemical devices (e.g. electrodes, separator) may react with the solute containing fluorine, to give hydrofluoric acid. When generated, the hydrofluoric acid reacts with the solvent in the electrolyte solution to give off gas. This phenomenon, for example in the case of a lithium-ion secondary battery, causes expansion of the cell, and accumulation of gas between electrodes which inhibits movement of lithium ions and lowers the battery performance.
Patent Literature 1 discloses adding into a non-aqueous electrolyte solution a benzenesulfonate ester that contains at least one fluorine atom and has a sulfonate ester structure. The addition is for production of an electrolyte solution for lithium secondary batteries which exhibits favorable initial cell capacity and cycle characteristics.
Patent Literature 2 discloses use of a C6 to C16 unsaturated hydrocarbon which is a compound containing carbon, fluorine, and hydrogen atoms and having at least one group with a carbon-carbon unsaturated bond, and which contains at least one fluorine atom substituted for a hydrogen atom bonding to a carbon atom. The use is for production of an electrolyte solution which is not likely to have a swollen cell because of decomposition gas in the electrolyte solution, and achieves excellent discharging and charging of the cell.
Also, sulfonate esters mentioned in Patent Literature 1 and Patent Literature 2 are known to go through reductive decomposition to form a film on a negative electrode, and have a possibility of being decomposed to emit gas on a positive electrode because of their low oxidation resistance.